Atrial tracking in an intracardiac ventricular pacemaker

ABSTRACT

An intracardiac ventricular pacemaker having a motion sensor is configured to produce a motion signal including an atrial systolic event and a ventricular diastolic event indicating a passive ventricular filling phase, set a detection threshold to a first amplitude during an expected time interval of the ventricular diastolic event and to a second amplitude lower than the first amplitude after an expected time interval of the ventricular diastolic event. The pacemaker is configured to detect the atrial systolic event in response to the motion signal crossing the detection threshold and set an atrioventricular pacing interval in response to detecting the atrial systolic event.

RELATED APPLICATION

This application is related to U.S. patent application Ser. No. ______,filed on even date herewith entitled “ATRIAL TRACKING IN AN INTRACARDIACVENTRICULAR PACEMAKER” (Attorney Docket Number C00013734.USU1), hereinincorporated by reference in its entirety.

TECHNICAL FIELD

The disclosure relates to an intracardiac ventricular pacemaker andassociated method for detecting atrial events from a motion sensorsignal and controlling atrial-synchronized ventricular pacing deliveredby the pacemaker.

BACKGROUND

Implantable cardiac pacemakers are often placed in a subcutaneous pocketand coupled to one or more transvenous medical electrical leads carryingpacing and sensing electrodes positioned in the heart. A cardiacpacemaker implanted subcutaneously may be a single chamber pacemakercoupled to one transvenous medical lead for positioning electrodes inone heart chamber, atrial or ventricular, or a dual chamber pacemakercoupled to two intracardiac leads for positioning electrodes in both anatrial and a ventricular chamber. Multi-chamber pacemakers are alsoavailable that may be coupled to three leads, for example, forpositioning electrodes for pacing and sensing in one atrial chamber andboth the right and left ventricles.

Intracardiac pacemakers have recently been introduced that areimplantable within a ventricular chamber of a patient's heart fordelivering ventricular pacing pulses. Such a pacemaker may sense R-wavesignals attendant to intrinsic ventricular depolarizations and deliverventricular pacing pulses in the absence of sensed R-waves. While singlechamber ventricular sensing and pacing by an intracardiac ventricularpacemaker may adequately address some patient conditions, otherconditions may require atrial and ventricular (dual chamber) sensing forproviding atrial-synchronized ventricular pacing in order to maintain aregular heart rhythm.

SUMMARY

In general, the disclosure is directed to a ventricular pacemaker andtechniques for detecting atrial systolic events from a motion sensorsignal for controlling atrial-synchronized ventricular pacing by theventricular pacemaker. A pacemaker operating according to the techniquesdisclosed herein sets a detection control parameters for detecting anatrial systolic event that occurs after a ventricular diastolic eventand for detecting an atrial systolic event that has become fused withthe ventricular diastolic event. The pacemaker sets an atrioventricularpacing interval in response to detecting atrial systolic events forproviding atrial-synchronized ventricular pacing.

In one example, the disclosure provides an intracardiac ventricularpacemaker, including a pulse generator, a motion sensor and a controlcircuit. The pulse generator is configured to generate and deliverpacing pulses to a ventricle of a patient's heart via electrodes coupledto the pacemaker. The motion sensor is configured to produce a motionsignal comprising an atrial systolic event and a ventricular diastolicevent indicating a passive ventricular filling phase. The controlcircuit is coupled to the motion sensor and the pulse generator and isconfigured to set a detection threshold to a first amplitude during anexpected time interval of the ventricular diastolic event and to asecond amplitude lower than the first amplitude after the expected timeinterval of the ventricular diastolic event. The control circuit detectsthe atrial systolic event in response to the motion signal crossing thedetection threshold, sets an atrioventricular pacing interval inresponse to detecting the atrial systolic event, and controls the pulsegenerator to deliver a pacing pulse to the ventricle in response to theatrioventricular pacing interval expiring.

In another example, the disclosure provides a method performed by anintracardiac pacemaker having a motion sensor. The method includesproducing by the motion sensor a motion signal comprising an atrialsystolic event and a ventricular diastolic event indicating a passiveventricular filling phase, setting a detection threshold to a firstamplitude during an expected time interval of the ventricular diastolicevent and to a second amplitude lower than the first amplitude after anexpected time interval of the ventricular diastolic event, detecting theatrial systolic event in response to the motion signal crossing thedetection threshold, setting an atrioventricular pacing interval inresponse to detecting the atrial systolic event, and delivering a pacingpulse to a ventricle of a patient's heart via electrodes coupled to thepacemaker in response to the atrioventricular pacing interval expiring.

In yet another example, the disclosure provides a non-transitorycomputer-readable medium storing a set of instructions which whenexecuted by a control circuit of an intracardiac ventricular pacemakerhaving a motion sensor, cause the pacemaker to produce by the motionsensor a motion signal comprising an atrial systolic event and aventricular diastolic event indicating a passive ventricular fillingphase, set a detection threshold to a first amplitude during an expectedtime interval of the ventricular diastolic event and to a secondamplitude lower than the first amplitude after an expected time intervalof the ventricular diastolic event, detect the atrial systolic event inresponse to the motion signal crossing the detection threshold, set anatrioventricular pacing interval in response to detecting the atrialsystolic event, and deliver a pacing pulse upon expiration of theatrioventricular pacing interval to a ventricle of a patient's heart viaelectrodes coupled to the pacemaker.

This summary is intended to provide an overview of the subject matterdescribed in this disclosure. It is not intended to provide an exclusiveor exhaustive explanation of the apparatus and methods described indetail within the accompanying drawings and description below. Furtherdetails of one or more examples are set forth in the accompanyingdrawings and the description below.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem that may be used to sense cardiac electrical signals and motionsignals induced by cardiac motion and flowing blood and provide pacingtherapy to a patient's heart.

FIG. 2A is a conceptual diagram of the intracardiac ventricularpacemaker shown in FIG. 1.

FIG. 2B is a conceptual diagram of another example of the intracardiacventricular pacemaker shown in FIG. 1.

FIG. 3 is a schematic diagram of an example configuration of thepacemaker of FIG. 2A.

FIG. 4 is an example of a motion sensor signal that may be acquired overa cardiac cycle by a motion sensor included in the ventricularintracardiac pacemaker of FIG. 1.

FIG. 5 is a flow chart of one method performed by an intracardiacventricular pacemaker for detecting an atrial systolic event from amotion sensor signal and controlling ventricular pacing.

FIG. 6 is an example of a motion sensor signal acquired over twodifferent ventricular cycles.

FIG. 7 is an averaged motion sensor signal.

FIG. 8 is a flow chart of a method performed by an intracardiacventricular pacemaker for detecting atrial events and controllingatrial-synchronized ventricular pacing according to another example.

FIG. 9 is a flow chart of a method for detecting atrial systolic eventsfrom a motion sensor signal for controlling atrial-synchronizedventricular pacing.

FIG. 10 is a timing diagram of a motion sensor signal that may bereceived by an intracardiac ventricular pacemaker.

FIG. 11 is a flow chart of a method for controlling atrial synchronizedventricular pacing by an intracardiac pacemaker according to anotherexample.

FIG. 12 is a flow chart of another example of a method for controllingatrial-synchronized ventricular pacing by an intracardiac pacemaker.

FIG. 13 is a flow chart of a method for controlling ventricular pacingby an intracardiac pacemaker according to yet another example.

DETAILED DESCRIPTION

FIG. 1 is a conceptual diagram illustrating an intracardiac pacingsystem 10 that may be used to sense cardiac electrical signals andmotion signals induced by cardiac motion and flowing blood and providepacing therapy to a patient's heart 8. IMD system 10 includes a rightventricular (RV) intracardiac pacemaker 14 and may optionally include aright atrial (RA) intracardiac pacemaker 12 in some examples. Pacemakers12 and 14 are transcatheter intracardiac pacemakers which may be adaptedfor implantation wholly within a heart chamber, e.g., wholly within theRV, wholly within the left ventricle (LV), wholly within the RA orwholly within the left atrium (LA) of heart 8.

In the example of FIG. 1, pacemaker 12 is positioned along anendocardial wall of the RA, e.g., along the RA lateral wall or RAseptum. Pacemaker 14 is positioned along an endocardial wall of the RV,e.g., near the RV apex though other locations are possible. Thetechniques disclosed herein are not limited to the pacemaker locationsshown in the example of FIG. 1 and other positions and relativelocations in the heart 8 and from each other are possible. For example,a ventricular intracardiac pacemaker 14 may be positioned in the LV forand configured to detect cardiac motion signals and deliveratrial-synchronized ventricular pacing to the LV using the techniquesdisclosed herein.

Pacemakers 12 and 14 are reduced in size compared to subcutaneouslyimplanted pacemakers and may be generally cylindrical in shape to enabletransvenous implantation via a delivery catheter. In other examples,pacemakers 12 and 14 may be positioned at any other location insideheart 8. For example, pacemaker 12 may be positioned outside or withinthe right atrium or left atrium to provide respective right atrial orleft atrial pacing. Pacemaker 14 may be positioned within the rightventricle or left ventricle to provide respective right ventricular orleft ventricular pacing and for sensing motion signals by a motionsensor within the ventricular chamber.

Pacemakers 12 and 14 are each capable of producing electricalstimulation pulses, e.g., pacing pulses, delivered to heart 8 via one ormore electrodes on the outer housing of the pacemaker. RA pacemaker 12is configured to sense a cardiac electrical signal from within the RAthat may be used to produce an RA intracardiac electrogram (EGM) signal.RV pacemaker 14 is configured to deliver RV pacing pulses and sense anRV cardiac electrical signal using housing based electrodes forproducing an RV EGM signal. The cardiac electrical signals may be sensedby the respective pacemaker 12 or 14 using the housing based electrodesthat are also used to deliver pacing pulses to the respective RA or RV.

In some examples, a patient may only require RV pacemaker 14 fordelivering ventricular pacing. In other examples, depending onindividual patient need, RA pacemaker 12 may be required for deliveringatrial pacing. The RV pacemaker 14 is configured to control the deliveryof ventricular pacing pulses to the RV in a manner that promotessynchrony between the RA activation and the RV activation, e.g., bymaintaining a target atrioventricular (AV) interval between atrialevents and ventricular pacing pulses. That is, the RV pacemaker 14controls RV pacing pulse delivery to maintain a desired AV intervalbetween atrial activations (intrinsic or pacing-evoked) corresponding toatrial systole and ventricular pacing pulses delivered to causeventricular depolarization.

According to the techniques described herein, atrial activations aredetected by RV pacemaker 14 from a motion sensor signal that includesmotion signals caused by ventricular and atrial events. For example,acceleration of blood flowing into the RV through the tricuspid valve 16between the RA and RV caused by atrial activation, sometimes referred toas the “atrial kick,” is detected by RV pacemaker 14 from the signalproduced by a motion sensor, for example an accelerometer, included inRV pacemaker 14. Other motion signals detected by RV pacemaker 14, suchas motion caused by ventricular contraction, motion caused byventricular relaxation, and motion caused by passive filling of theventricle are described below in conjunction with FIG. 4.

Atrial P-waves that are attendant to atrial depolarization arerelatively low amplitude signals in the near-field RV cardiac electricalsignal received by pacemaker 14 (e.g., compared to the near-fieldR-wave) and therefore can be difficult to reliably detect from thecardiac electrical signal acquired by RV pacemaker 14. As such,atrial-synchronized ventricular pacing by RV pacemaker 14 may not bereliable when based solely on a cardiac electrical signal received by RVpacemaker 14. According to the techniques disclosed herein, the RVpacemaker 14 includes a motion sensor, such as an accelerometer, and isconfigured to detect an atrial event corresponding to atrial mechanicalactivation or atrial systole using a signal from the motion sensor.Ventricular pacing pulses are synchronized to the atrial event that isdetected from the accelerometer signal by setting a programmableatrioventricular (AV) pacing interval that controls the timing of theventricular pacing pulse relative to the detected atrial systolic event.As described below, detection of the atrial systolic event used tosynchronize ventricular pacing pulses to atrial systole may includedetection of other cardiac event motion signals in order to positivelyidentify the atrial systolic event.

A target AV interval may be a programmed value selected by a clinicianand is the time interval from the detection of the atrial event untildelivery of the ventricular pacing pulse. In some instances, the targetAV interval may be started from the time the atrial systolic event isdetected based on a motion sensor signal or starting from an identifiedfiducial point of the atrial event signal. The target AV interval may beidentified as being hemodynamically optimal for a given patient based onclinical testing or assessments of the patient or based on clinical datafrom a population of patients. The target AV interval may be determinedto be optimal based on relative timing of electrical and mechanicalevents as identified from the cardiac electrical signal received by RVpacemaker 14 and the motion sensor signal received by RV pacemaker 14.

Pacemakers 12 and 14 may each be capable of bidirectional wirelesscommunication with an external device 20 for programming the AV pacinginterval and other pacing control parameters as well as mechanical eventsensing parameters utilized for detecting ventricular mechanical eventsand the atrial systolic event from the motion sensor signal. Aspects ofexternal device 20 may generally correspond to the externalprogramming/monitoring unit disclosed in U.S. Pat. No. 5,507,782(Kieval, et al.), hereby incorporated herein by reference in itsentirety. External device 20 is often referred to as a “programmer”because it is typically used by a physician, technician, nurse,clinician or other qualified user for programming operating parametersin pacemakers 12 and 14. External device 20 may be located in a clinic,hospital or other medical facility. External device 20 may alternativelybe embodied as a home monitor or a handheld device that may be used in amedical facility, in the patient's home, or another location. Operatingparameters, including sensing and therapy delivery control parameters,may be programmed into pacemakers 12 and 14 using external device 20.

External device 20 is configured for bidirectional communication withimplantable telemetry circuitry included in RV pacemaker 14 and RApacemaker 12 (when present). External device 20 establishes a wirelessradio frequency (RF) communication link 22 with RA pacemaker 12 andwireless RF communication link 24 with RV pacemaker 14 using acommunication protocol that appropriately addresses the targetedpacemaker 12 or 14. Communication links 22 and 24 may be establishedusing an RF link such as BLUETOOTH®, Wi-Fi, Medical ImplantCommunication Service (MICS) or other communication bandwidth. In someexamples, external device 20 may include a programming head that isplaced proximate pacemaker 12 or 14 to establish and maintain acommunication link, and in other examples external device 20 andpacemakers 12 and 14 may be configured to communicate using a distancetelemetry algorithm and circuitry that does not require the use of aprogramming head and does not require user intervention to maintain acommunication link. An example RF telemetry communication system thatmay be implemented in system 10 is generally disclosed in U.S. Pat. No.5,683,432 (Goedeke, et al.), hereby incorporated herein by reference inits entirety.

External device 20 may display data and information relating topacemaker functions to a user for reviewing pacemaker operation andprogrammed parameters as well as EGM signals transmitted from pacemaker14 or pacemaker 12, motion sensor signals acquired by pacemaker 14, orother physiological data that is acquired by and retrieved frompacemakers 12 and/or 14 during an interrogation session.

It is contemplated that external device 20 may be in wired or wirelessconnection to a communications network via a telemetry circuit thatincludes a transceiver and antenna or via a hardwired communication linefor transferring data to a remote database or computer to allow remotemanagement of the patient.

Remote patient management systems including a remote patient databasemay be configured to utilize the presently disclosed techniques toenable a clinician to review EGM, motion sensor, and marker channel dataand authorize programming of sensing and therapy control parameters inRV pacemaker 14, e.g., after viewing a visual representation of EGM,motion sensor signal and marker channel data.

Pacemaker 12 and pacemaker 14 may or may not be configured tocommunicate directly with each other. When pacemakers 12 and 14 areconfigured to communicate with each other, communication may beminimized in order to conserve battery life of the intracardiacpacemakers 12 and 14. As such, communication may not occur on abeat-by-beat basis between the RA pacemaker 12 and RV pacemaker 14 forcommunicating when the other pacemaker is sensing cardiac events or whenit is delivering pacing pulses. As disclosed herein, RV pacemaker 14,however, is configured to detect atrial events as often as beat-by-beatfrom a motion sensor signal, without requiring communication signalsfrom RA pacemaker 12 to provide atrial event detection for controllingatrial-synchronized ventricular pacing.

FIG. 2A is a conceptual diagram of the intracardiac RV pacemaker 14shown in FIG. 1. RV pacemaker 14 includes electrodes 162 and 164 spacedapart along the housing 150 of pacemaker 14 for sensing cardiacelectrical signals and delivering pacing pulses. Electrode 164 is shownas a tip electrode extending from a distal end 102 of pacemaker 14, andelectrode 162 is shown as a ring electrode along a mid-portion ofhousing 150, for example adjacent proximal end 104. Distal end 102 isreferred to as “distal” in that it is expected to be the leading end aspacemaker 14 is advanced through a delivery tool, such as a catheter,and placed against a targeted pacing site.

Electrodes 162 and 164 form an anode and cathode pair for bipolarcardiac pacing and sensing. In alternative embodiments, pacemaker 14 mayinclude two or more ring electrodes, two tip electrodes, and/or othertypes of electrodes exposed along pacemaker housing 150 for deliveringelectrical stimulation to heart 8 and sensing cardiac electricalsignals. Electrodes 162 and 164 may be, without limitation, titanium,platinum, iridium or alloys thereof and may include a low polarizingcoating, such as titanium nitride, iridium oxide, ruthenium oxide,platinum black among others. Electrodes 162 and 164 may be positioned atlocations along pacemaker 14 other than the locations shown.

Housing 150 is formed from a biocompatible material, such as a stainlesssteel or titanium alloy. In some examples, the housing 150 may includean insulating coating. Examples of insulating coatings include parylene,urethane, PEEK, or polyimide among others. The entirety of the housing150 may be insulated, but only electrodes 162 and 164 uninsulated.Electrode 164 may serve as a cathode electrode and be coupled tointernal circuitry, e.g., a pacing pulse generator and cardiacelectrical signal sensing circuitry, enclosed by housing 150 via anelectrical feedthrough crossing housing 150. Electrode 162 may be formedas a conductive portion of housing 150 as a ring electrode that iselectrically isolated from the other portions of the housing 150 asgenerally shown in FIG. 2A. In other examples, the entire periphery ofthe housing 150 may function as an electrode that is electricallyisolated from tip electrode 164, instead of providing a localized ringelectrode such as anode electrode 162. Electrode 162 formed along anelectrically conductive portion of housing 150 serves as a return anodeduring pacing and sensing.

The housing 150 includes a control electronics subassembly 152, whichhouses the electronics for sensing cardiac signals, producing pacingpulses and controlling therapy delivery and other functions of pacemaker14 as described below in conjunction with FIG. 3. A motion sensor may beimplemented as an accelerometer enclosed within housing 150 in someexamples. The accelerometer provides a signal to a processor included incontrol electronics subassembly 152 for signal processing and analysisfor detecting ventricular mechanical events and atrial systolic eventsfor timing ventricular pacing pulses as described below.

Housing 150 further includes a battery subassembly 160, which providespower to the control electronics subassembly 152. Battery subassembly160 may include features of the batteries disclosed in commonly-assignedU.S. Pat. No. 8,433,409 (Johnson, et al.) and U.S. Pat. No. 8,541,131(Lund, et al.), both of which are hereby incorporated by referenceherein in their entirety.

Pacemaker 14 may include a set of fixation tines 166 to secure pacemaker14 to patient tissue, e.g., by actively engaging with the ventricularendocardium and/or interacting with the ventricular trabeculae. Fixationtines 166 are configured to anchor pacemaker 14 to position electrode164 in operative proximity to a targeted tissue for deliveringtherapeutic electrical stimulation pulses. Numerous types of activeand/or passive fixation members may be employed for anchoring orstabilizing pacemaker 14 in an implant position. Pacemaker 14 mayinclude a set of fixation tines as disclosed in commonly-assigned,pre-grant publication U.S. 2012/0172892 (Grubac, et al.), herebyincorporated herein by reference in its entirety.

Pacemaker 14 may optionally include a delivery tool interface 158.Delivery tool interface 158 may be located at the proximal end 104 ofpacemaker 14 and is configured to connect to a delivery device, such asa catheter, used to position pacemaker 14 at an implant location duringan implantation procedure, for example within a heart chamber.

FIG. 2B is a conceptual diagram of another example of RV pacemaker 14.In FIG. 2B, RV pacemaker 14 includes a proximal sensing extension 165extending away from housing 150 and carrying a pair of sensingelectrodes 167 and 168. The proximal sensing extension 165 may becoupled to the housing 150 for positioning a return sensing electrode168 or 167 which may be paired with distal electrode 164 at an increasedinter-electrode distance compared to the inter-electrode spacing ofhousing-based electrodes 162 and 164. The increased inter-electrodedistance may facilitate sensing of far-field atrial signals such asP-waves attendant to atrial depolarization.

Alternatively, electrodes 167 and 168 may form a sensing electrode pairfor sensing atrial P-waves. When distal end 102 is fixed along the RVapex, sensing extension 165 may extend toward the RA thereby positioningelectrodes 167 and 168 nearer the atrial tissue for sensing far-fieldatrial P-waves. One electrode 167 may be coupled to sensing circuitryenclosed in housing 150 via an electrical feedthrough crossing housing150, and one electrode 168 may be coupled to housing 150 to serve as aground electrode.

FIG. 3 is a schematic diagram of an example configuration of pacemaker14 shown in FIG. 1. Pacemaker 14 includes a pulse generator 202, asensing circuit 204, a control circuit 206, memory 210, telemetrycircuit 208, motion sensor 212 and a power source 214. Motion sensor 212is implemented as an accelerometer in the examples described herein andmay also be referred to herein as “accelerometer 212.” Motion sensor 212is not limited to being an accelerometer, however, and other motionsensors may be utilized successfully in pacemaker 14 for detectingcardiac motion signals according to the techniques described herein.Examples of motion sensors that may be implemented in pacemaker 14include piezoelectric sensors and micro electro-mechanical systems(MEMS) devices.

Motion sensor 212 may be a multi-axis sensor, e.g., a two-dimensional orthree-dimensional sensor, with each axis providing a signal that may beanalyzed individually or in combination for detecting cardiac mechanicalevents. Motion sensor 212 produces an electrical signal correlated tomotion or vibration of sensor 212 (and pacemaker 14), e.g., whensubjected to flowing blood and cardiac motion. Motion sensor 212 may bea one-dimensional, single axis accelerometer, two-dimensional orthree-dimensional multi-axis accelerometer. One example of anaccelerometer for use in implantable medical devices is generallydisclosed in U.S. Pat. No. 5,885,471 (Ruben, et al.), incorporatedherein by reference in its entirety. An implantable medical devicearrangement including a piezoelectric accelerometer for detectingpatient motion is disclosed, for example, in U.S. Pat. No. 4,485,813(Anderson, et al.) and U.S. Pat. No. 5,052,388 (Sivula, et al.), both ofwhich patents are hereby incorporated by reference herein in theirentirety. Examples of three-dimensional accelerometers that may beimplemented in pacemaker 14 and used for detecting cardiac mechanicalevents using the presently disclosed techniques are generally describedin U.S. Pat. No. 5,593,431 (Sheldon) and U.S. Pat. No. 6,044,297(Sheldon), both of which are incorporated herein by reference in theirentirety. Other accelerometer designs may be used for producing anelectrical signal that is correlated to motion imparted on pacemaker 14due to ventricular and atrial events.

The various circuits represented in FIG. 3 may be combined on one ormore integrated circuit boards which include a specific integratedcircuit (ASIC), an electronic circuit, a processor (shared, dedicated,or group) and memory that execute one or more software or firmwareprograms, a combinational logic circuit, state machine or other suitablecomponents that provide the described functionality.

Sensing circuit 204 is configured to receive a cardiac electrical signalvia electrodes 162 and 164 by a pre-filter and amplifier circuit 220.Pre-filter and amplifier circuit may include a high pass filter toremove DC offset, e.g., a 2.5 to 5 Hz high pass filter, or a widebandfilter having a passband of 2.5 Hz to 100 Hz to remove DC offset andhigh frequency noise. Pre-filter and amplifier circuit 220 may furtherinclude an amplifier to amplify the “raw” cardiac electrical signalpassed to analog-to-digital converter (ADC) 226. ADC 226 may pass amulti-bit, digital electrogram (EGM) signal to control circuit 206 foruse by atrial event detector circuit 240 in identifying ventricularelectrical events (e.g., R-waves or T-waves) and/or atrial electricalevents, e.g., P-waves. Identification of cardiac electrical events maybe used in algorithms for detecting atrial systolic events from themotion sensor signal. The digital signal from ADC 226 may be passed torectifier and amplifier circuit 222, which may include a rectifier,bandpass filter, and amplifier for passing a cardiac signal to R-wavedetector 224.

R-wave detector 224 may include a sense amplifier or other detectioncircuitry that compares the incoming rectified, cardiac electricalsignal to an R-wave detection threshold, which may be an auto-adjustingthreshold. When the incoming signal crosses the R-wave detectionthreshold, the R-wave detector 224 produces an R-wave sensed eventsignal (R-sense) that is passed to control circuit 206. In otherexamples, R-wave detector 224 may receive the digital output of ADC 226for detecting R-waves by a comparator, morphological signal analysis ofthe digital EGM signal or other R-wave detection techniques. R-wavesensed event signals passed from R-wave detector 224 to control circuit206 may be used for scheduling ventricular pacing pulses by pace timingcircuit 242 and for use in identifying the timing of ventricularelectrical events in algorithms performed by atrial event detectorcircuit 240 for detecting atrial systolic events from a signal receivedfrom motion sensor 212.

Control circuit 206 includes an atrial event detector circuit 240, pacetiming circuit 242, and processor 244. Atrial event detector circuit 240is configured to detect atrial mechanical events from a signal receivedfrom motion sensor 212. As described below, one or more ventricularmechanical events may be detected from the motion sensor signal in agiven cardiac cycle to facilitate positive detection of the atrialsystolic event from the motion sensor signal during the ventricularcycle.

Control circuit 206 may receive R-wave sensed event signals and/ordigital cardiac electrical signals from sensing circuit 204 for use indetecting and confirming cardiac events and controlling ventricularpacing. For example, R-wave sensed event signals may be passed to pacetiming circuit 242 for inhibiting scheduled ventricular pacing pulses orscheduling ventricular pacing pulses when pacemaker 14 is operating in anon-atrial tracking ventricular pacing mode. R-wave sensed event signalsmay also be passed to atrial event detector circuit 240 for use insetting ventricular event detection windows and/or atrial eventrefractory periods, for example as shown and described in conjunctionwith FIG. 6.

Atrial event detector circuit 240 receives a motion signal from motionsensor 212 and starts an atrial refractory period in response to aventricular electrical event, e.g., an R-wave sensed event signal fromsensing circuit 204 or delivery of a pacing pulse by pulse generator202. Atrial event detector circuit 240 determines if the motion sensorsignal satisfies atrial mechanical event detection criteria outside ofthe refractory period. The motion sensor signal during the refractoryperiod may be monitored by atrial event detector circuit 240 for thepurposes of detecting ventricular mechanical events, which may be usedfor confirming or validating atrial systolic event detection and/orsetting atrial systolic event detection control parameters as furtherdescribed below, e.g., in conjunction with FIG. 10. As such, ventricularmechanical event detection windows may be set during the atrialrefractory period and may be set according to predetermined timeintervals following identification of a ventricular electrical event.Atrial event detector circuit 240 may be configured to detect one ormore ventricular mechanical events during respective ventricular eventdetection windows during the atrial refractory period. The timing anddetection of the ventricular mechanical events may be used to update theatrial refractory period and/or an atrial systolic detection thresholdamplitude and may be used to confirm detection of the atrial systolicevent occurring subsequent to expected ventricular mechanical events.

Atrial event detector circuit 240 passes an atrial event detectionsignal to processor 244 and/or pace timing circuit 242. Pace timingcircuit 242 (or processor 244) may additionally receive R-wave sensedevent signals from R-wave detector 224 for use in controlling the timingof pacing pulses delivered by pulse generator 202. Processor 244 mayinclude one or more clocks for generating clock signals that are used bypace timing circuit 242 to time out an AV pacing interval that isstarted upon receipt of an atrial event detection signal from atrialevent detector circuit 240. Pace timing circuit 242 may include one ormore pacing escape interval timers or counters that are used to time outthe AV pacing interval, which may be a programmable interval stored inmemory 210 and retrieved by processor 244 for use in setting the AVpacing interval used by pace timing circuit 242.

Pace timing circuit 242 may additionally include a lower pacing rateinterval timer for controlling a minimum ventricular pacing rate. Forexample, if an atrial systolic event is not detected from the motionsensor signal triggering a ventricular pacing pulse at the programmed AVpacing interval, a ventricular pacing pulse may be delivered by pulsegenerator 202 upon expiration of the lower pacing rate interval toprevent ventricular asystole and maintain a minimum ventricular rate.

Processor 244 may retrieve other programmable pacing control parameters,such as pacing pulse amplitude and pacing pulse width that are passed topulse generator 202 for controlling pacing pulse delivery. In additionto providing control signals to pace timing circuit 242 and pulsegenerator 202 for controlling pacing pulse delivery, processor 244 mayprovide sensing control signals to sensing circuit 204, e.g., R-wavesensing threshold, sensitivity, various blanking and refractoryintervals applied to the cardiac electrical signal, and atrial eventdetection control signals to atrial event detector circuit 240 for usein detecting and confirming atrial systolic events, e.g., ventricularevent detection windows, atrial refractory period, detection thresholdamplitudes applied to the motion sensor signal, and any other atrialevent detection criteria applied by circuitry included in atrial eventdetector circuit 240.

The functions attributed to pacemaker 14 herein may be embodied as oneor more processors, controllers, hardware, firmware, software, or anycombination thereof. Depiction of different features as specificcircuitry is intended to highlight different functional aspects and doesnot necessarily imply that such functions must be realized by separatehardware, firmware or software components or by any particular circuitarchitecture. Rather, functionality associated with one or more circuitsdescribed herein may be performed by separate hardware, firmware orsoftware components, or integrated within common hardware, firmware orsoftware components. For example, atrial systolic event detection fromthe motion sensor signal and ventricular pacing control operationsperformed by pacemaker 14 may be implemented in control circuit 206executing instructions stored in memory 210 and relying on input fromsensing circuit 204 and motion sensor 212.

The operation of circuitry included in pacemaker 14 as disclosed hereinshould not be construed as reflective of a specific form of hardware,firmware and software necessary to practice the techniques described. Itis believed that the particular form of software, hardware and/orfirmware will be determined primarily by the particular systemarchitecture employed in the pacemaker 14 and by the particular sensingand therapy delivery circuitry employed by the pacemaker 14. Providingsoftware, hardware, and/or firmware to accomplish the describedfunctionality in the context of any modern pacemaker, given thedisclosure herein, is within the abilities of one of skill in the art.

Pulse generator 202 generates electrical pacing pulses that aredelivered to the RV of the patient's heart via cathode electrode 164 andreturn anode electrode 162. Pulse generator 202 may include chargingcircuit 230, switching circuit 232 and an output circuit 234 Chargingcircuit 230 may include a holding capacitor that may be charged to apacing pulse amplitude by a multiple of the battery voltage signal ofpower source 214 under the control of a voltage regulator. The pacingpulse amplitude may be set based on a control signal from controlcircuit 206. Switching circuit 232 may control when the holdingcapacitor of charging circuit 230 is coupled to the output circuit 234for delivering the pacing pulse. For example, switching circuit 232 mayinclude a switch that is activated by a timing signal received from pacetiming circuit 242 upon expiration of an AV pacing interval (or lowerrate pacing interval) and kept closed for a programmed pacing pulseduration to enable discharging of the holding capacitor of chargingcircuit 230. The holding capacitor, previously charged to the pacingpulse voltage amplitude, is discharged across electrodes 162 and 164through the output capacitor of output circuit 234 for the programmedpacing pulse duration. Examples of pacing circuitry generally disclosedin U.S. Pat. No. 5,507,782 (Kieval, et al.) and in commonly assignedU.S. Pat. No. 8,532,785 (Crutchfield, et al.), both of which patents areincorporated herein by reference in their entirety, may be implementedin pacemaker 14 for charging a pacing capacitor to a predeterminedpacing pulse amplitude under the control of control circuit 206 anddelivering a pacing pulse.

Memory 210 may include computer-readable instructions that, whenexecuted by control circuit 206, cause control circuit 206 to performvarious functions attributed throughout this disclosure to pacemaker 14.The computer-readable instructions may be encoded within memory 210.Memory 210 may include any non-transitory, computer-readable storagemedia including any volatile, non-volatile, magnetic, optical, orelectrical media, such as a random access memory (RAM), read-only memory(ROM), non-volatile RAM (NVRAM), electrically-erasable programmable ROM(EEPROM), flash memory, or other digital media with the sole exceptionbeing a transitory propagating signal. Memory 210 may store timingintervals and other data used by control circuit 206 to control thedelivery of pacing pulses by pulse generator 202, e.g., by detecting anatrial systolic event by atrial event detector circuit 240 from themotion sensor signal and setting a pacing escape interval timer includedin pace timing circuit 242, according to the techniques disclosedherein.

Power source 214 provides power to each of the other circuits andcomponents of pacemaker 14 as required. Control circuit 206 may executepower control operations to control when various circuits or componentsare powered to perform various pacemaker functions. Power source 214 mayinclude one or more energy storage devices, such as one or morerechargeable or non-rechargeable batteries. The connections betweenpower source 214 and other pacemaker circuits and components are notshown in FIG. 3 for the sake of clarity.

Telemetry circuit 208 includes a transceiver 209 and antenna 211 fortransferring and receiving data via a radio frequency (RF) communicationlink. Telemetry circuit 208 may be capable of bi-directionalcommunication with external device 20 (FIG. 1) as described above.Motion sensor signals and cardiac electrical signals, and/or dataderived therefrom may be transmitted by telemetry circuit 208 toexternal device 20. Programmable control parameters and algorithms forperforming atrial event detection and ventricular pacing control may bereceived by telemetry circuit 208 and stored in memory 210 for access bycontrol circuit 206.

FIG. 4 is an example of a motion sensor signal 250 that may be acquiredby motion sensor 212 over a cardiac cycle. Vertical dashed lines 252 and262 denote the timing of two consecutive ventricular events (anintrinsic ventricular depolarization or a ventricular pace), marking therespective beginning and end of the ventricular cycle 251. The motionsignal includes an A1 event 254, an A2 event 256, an A3 event 258 and anA4 event 260. The A1 event 254 is an acceleration signal (in thisexample when motion sensor 250 is implemented as an accelerometer) thatoccurs during ventricular contraction and marks the approximate onset ofventricular mechanical systole. The A1 event is also referred to hereinas a “ventricular contraction event.” The A2 event 265 is anacceleration signal that occurs during ventricular relaxation and marksthe approximate offset or end of ventricular mechanical systole. The A2event is also referred to herein as the “ventricular relaxation event.”The A3 event 258 is an acceleration signal that occurs during passiveventricular filling and marks ventricular mechanical diastole. The A3event is also referred to herein as the “ventricular passive fillingevent.” Since the A2 event occurs with the end of ventricular systole,it is an indicator of the onset of ventricular diastole. The A3 eventoccurs during ventricular diastole. As such, the A2 and A3 events may becollectively referred to as ventricular mechanical diastolic eventsbecause they are both indicators of the ventricular diastolic period.

The A4 event 260 is an acceleration signal that occurs during atrialcontraction and active ventricular filling and marks atrial mechanicalsystole. The A4 event 260 is also referred to herein as the “atrialsystolic event” or merely the “atrial event,” and is the atrial systolicevent that is detected from motion sensor signal 250 by atrial eventdetector circuit 240 for controlling pace timing circuit 242 to triggerventricular pacing pulse delivery by starting an AV pacing interval inresponse to detecting the A4 event 260. As described below, controlcircuit 206 may be configured to detect one or more of the A1, A2, andA3 events from motion sensor signal 250, for at least some ventricularcardiac cycles, for use in positively detecting the A4 event 260 andsetting atrial event detection control parameters. The A1, A2 and/or A3events may be detected and characterized to avoid false detection of A4events and promote reliable A4 event detection for proper timing ofatrial-synchronized ventricular pacing pulses.

FIG. 5 is a flow chart 300 of one method performed by pacemaker 14 fordetecting the A4 event and controlling ventricular pacing. At block 302,control circuit 206 identifies a ventricular event. The ventricularevent may be an R-wave sensed event signal received from sensing circuit204 or a ventricular pacing pulse delivered by pulse generator 202.Since the ventricular A1, A2 and A3 events may have differentcharacteristics during an intrinsic ventricular rhythm than during aventricular paced rhythm, the methods described herein for determiningamplitudes, time intervals or other characteristics of the A1, A2 and A3events for use in setting A4 detection control parameters or confirmingA4 event detection may be determined for both an intrinsic ventricularrhythm and a ventricular paced rhythm.

For example, as described in conjunction with the flow charts and timingdiagrams presented herein, various time intervals, sensing windows,atrial refractory period, and atrial event detection threshold amplitudemay be set based on characterizations of one or more of the A1, A2 andA3 events. One set of A4 detection control parameters andcharacteristics of the A1, A2 and A3 events may be determined and storedfor use during episodes of ventricular sensing (ventricular intrinsicrhythm), and another set of A4 detection control parameters andcharacteristics of the A1, A2 and A3 events may be determined and storedfor used during episodes of ventricular pacing.

During ventricular sensing, control circuit 206 may be configured todiscriminate a normal sinus R-wave from a premature ventricularcontraction (PVC) so that ventricular events identified at block 302 foruse in starting a search for the A1 through A4 events from the motionsensor signal do not include PVCs. When a ventricular event, sensed orpaced, is identified at block 302 that is not a PVC, pace timing circuit242 may set an escape interval timer to a lower rate (LR) pacinginterval. If the lower rate pacing interval expires (as described belowin conjunction with block 315), a ventricular pacing pulse may bedelivered, asynchronous to atrial activity, in order to maintain someminimum, base ventricular rate.

At block 304, atrial event detector 240 detects the A1 through A3 motionsignals. Briefly, atrial event detector 240 may compare the motionsensor signal to one or more pre-determined detection thresholdamplitudes during one or more time windows set in response toidentifying the ventricular event at bock 302 for detecting the A1through A3 events. In some examples, the A4 event may also be detectedat block 304 to increase confidence in the positive identification ofeach of the four motion sensor signals A1 through A4 in a given cardiaccycle. In this example, the A1 through A3 events, and optionally A4, maybe detected on a beat-by-beat basis.

After the A1 through A3 events are detected, the A1-A3 time interval isdetermined at block 304 as the time interval from the A1 event detectionto the A3 event detection. The A1-A3 time interval may be used to updatean A1-A3 interval trend at block 308. For example, a running averageA1-A3 time interval may be updated at block 308 using the most recent NA1-A3 time interval measurements, e.g., the most recent three to twelveA1-A3 time intervals.

The A1-A3 time interval is used to set a post-ventricular atrialrefractory period at block 310. This atrial refractory period is alsoreferred to herein as an “A4 refractory period” because A4 eventdetection is inhibited during the atrial refractory period. When aventricular event is identified at block 302, atrial event detector 240may start the atrial refractory period at block 310. The atrialrefractory period may be set to a percentage longer than or a fixedinterval longer than the A1-A3 time interval. For example, the atrialrefractory period may be set to be 50 to 150 ms longer than the A1-A3time interval, though shorter or longer fixed intervals may be added tothe A1-A3 time interval for setting the atrial refractory period. Thefixed time interval used to set the atrial refractory period may varydepending on heart rate in some examples.

During the atrial refractory period, any motion sensor events that aredetected, or cross a detection threshold amplitude, are ignored for thepurposes of triggering a ventricular pacing pulse and starting an AVpacing interval. Ventricular mechanical events A1 through A3 may bedetected during the atrial refractory period, as indicated at block 304,to determine the A1-A3 time interval and update the A1-A3 interval trend(blocks 306 and 308), either periodically or on a beat-by-beat basis.

At block 312, atrial event detector circuit 240 monitors the motionsensor signal to detect the A4 event after the expiration of the atrialrefractory period. If the A4 event is not detected before the lowerpacing rate interval expires (block 315), a ventricular pacing pulse isdelivered at block 316 to ensure a minimum ventricular rate, e.g., 40 to60 beats per minute. Furthermore, it is to be understood that if anintrinsic R-wave is sensed before an A4 event is detected, the processof FIG. 5 may return to block 302 where the sensed R-wave is identifiedas a ventricular electrical event and control circuit 206 restarts theprocess of detecting the A4 event on the next ventricular cycle.

If the A4 event is detected before the lower pacing rate intervalexpires, control circuit 206 sets the AV pacing interval at block 314 inresponse to detecting the A4 event. If an intrinsic R-wave is not sensedfrom the cardiac electrical signal by sensing circuit 204 during the AVpacing interval, “no” branch of block 316, a ventricular pacing pulse isdelivered by pulse generator 202 at block 318 upon expiration of the AVpacing interval. The ventricular pacing pulse, if delivered, andotherwise the sensed R-wave, is identified as the next ventricular eventat block 302 and the process repeats.

In this way, the A1 through A3 events may be detected from the motionsensor signal on a beat-by-beat (or less frequent) basis for updatingthe A1-A3 time interval trend used to set the atrial refractory periodto provide a high likelihood of positively detecting the A4 event andproperly timing a ventricular pacing pulse in synchrony with the atrialevent. Other motion sensor signal events A1 through A3 are unlikely tobe falsely detected as the A4 event by applying the atrial refractoryperiod set based on the A1-A3 timing.

In some examples, rather than determining an A1-A3 time interval, a timeinterval to the A2 event may be determined so that the atrial refractoryperiod is set based on the A1-A2 time interval to extend through atleast the A2 event and expire before the A3 event. In this example, anA4 detection threshold amplitude may be set higher than an expected A3event amplitude to allow detection of the A4 event earlier in theventricular cycle, for example as the atrial rate is increasing. Inother cases, the time interval from the identified ventricularelectrical event to the A1, A2 or A3 event may be determined and used insetting the atrial refractory period.

In some examples, the process of blocks 304 through 308 is performedperiodically rather than on a beat-by-beat basis. For example detectionof A1-A3 events during the atrial refractory period may occur on everythird cardiac cycle, every eighth cardiac cycle, once a minute or otherpredetermined schedule for updating the A1-A3 time interval (or otherventricular event time interval as discussed above) used for setting theatrial refractory period at block 310. In some cases, the heart rate,paced or intrinsic, may be monitored and the A1-A3 events may bedetected for updating the A1-A3 interval trend when the heart ratechanges by more than a predetermined amount. For example, ventricularevent intervals between consecutive ventricular events may be determinedupon identifying ventricular events at block 302. The ventricular eventintervals may be RR intervals between consecutively sensed intrinsicR-waves or VV intervals between consecutively delivered ventricularpacing pulses and may include RV intervals between a sensed intrinsicR-wave and a consecutively delivered pacing pulse and VR intervalsbetween a delivered pacing pulse and a consecutively sensed R-wave. Boththe intrinsic heart rate and the paced rate may change, e.g., whenpacemaker 14 is a rate responsive pacemaker. If the ventricular eventinterval changes or a trend in the ventricular event interval changes bymore than a predetermined amount, the control circuit may perform blocks304 through 308 to update the A1-A3 interval trend used for setting theatrial refractory period.

In other examples, if the A4 event is not detected at block 312 afterthe atrial refractory period and before the next ventricular event(intrinsic or paced) is identified at block 302, the control circuit 206may perform the process of blocks 304 through 306 for a predeterminednumber of consecutive or non-consecutive cardiac cycles to update theA1-A3 interval trend used to set the atrial refractory period to restoreA4 detection.

FIG. 6 is an example of a motion sensor signals 400 and 410 acquiredover two different cardiac cycles. A ventricular pacing pulse isdelivered at time 0.0 seconds for both cardiac cycles. The top sensorsignal 400 is received over one cardiac cycle and the bottom sensorsignal 401 is received over a different cardiac cycle. The two signals400 and 410 are aligned in time at 0.0 seconds, the time of theventricular pacing pulse delivery.

The A1 events 402 and 412 of the respective motion sensor signals 400and 410, which occur during ventricular contraction, are observed to bewell-aligned in time following the ventricular pacing pulse at time 0.0seconds. Similarly, the A2 events 404 and 414 (occurring duringventricular relaxation) and the A3 events 406 and 416 (occurring duringpassive ventricular filling) are well-aligned in time. Since the A1, A2and A3 events are ventricular events, occurring during ventricularcontraction, ventricular relaxation, and passive ventricular filling,respectively, these events are expected to occur at relativelyconsistent intervals following a ventricular electrical event, theventricular pacing pulse in this example, and relative to each other.The time relationship of the A1, A2 and A3 events may be differentfollowing a ventricular pacing pulse compared to following a sensedintrinsic R-wave, however, during a stable paced or intrinsicventricular rhythm, the relative timing of A1, A2 and A3 events to eachother and the immediately preceding ventricular electrical event isexpected to be consistent.

The A4 events 408 and 418 of the first and second motion sensor signals400 and 410 respectively are not aligned in time. The A4 event occursduring atrial systole and as such the time interval of the A4 eventfollowing the immediately preceding ventricular electrical event (sensedR-wave or ventricular pacing pulse) and the preceding A1 through A3events may vary between cardiac cycles.

The consistency of the timing of the A1 through A3 events relative toeach other and the immediately preceding ventricular electrical eventmay be used for determining the atrial refractory period and increasingconfidence in reliably detecting A4 events 408 and 418. In someexamples, an A1 sensing window 420 may be set based on an expectedVpace-A1 time interval. The Vpace-A1 time interval 430 may be measuredwhen the motion sensor signal 400 or 410 crosses an A1 sensing thresholdamplitude 440. The A1 sensing window 420 may be adjusted on the nextcardiac cycle based on the Vpace-A1 time interval 430 determined on thecurrent cardiac cycle or a running average Vpace-A1 time interval.

An A2 sensing window 422 may be set based on an expected Vpace-A2 timeinterval (not explicitly shown but understood to be the total time from0.0 seconds to an A2 event detection) or an A1-A2 time interval 432(time from A1 detection to time of A2 detection). The A2 event 404 or414 may be detected at the time of the first positive-going crossing ofan A2 sensing threshold amplitude 442 by the motion sensor signal 400 or410 during the A2 sensing window 422. The A2 sensing window 422 may beadjusted on the next cardiac cycle based on the Vpace-A2 time intervalor A1-A2 time interval 432 determined on the current cardiac cycle.

Similarly, an A3 sensing window 424 may be set based on an expectedVpace-A3 time interval (not explicitly labeled but understood to be sumof time intervals 430 and 434), A1-A3 time interval 434, or A2-A3 timeinterval (not explicitly labeled but understood to be the time intervalfrom the sensed A2 event 404 or 414 to the sensed A3 event 406 or 416).The A3 event 406 or 416 may be detected during the A3 sensing window 424when the motion sensor signal 400 or 410, respectively, crosses an A3sensing threshold amplitude 444. The A3 sensing window 424 may beadjusted on the next cardiac cycle based on the Vpace-A3 time interval,A1-A3 time interval 434, or the A2-A3 time interval determined duringthe current cardiac cycle.

Each of the sensing windows 420, 422 and 424 may be set based on ahistory of time intervals determined from a ventricular pacing pulse orsensed intrinsic R-wave to the respective A1 event 402 or 412, A2 event404 or 414 and A3 event 406 or 416 or based on a history of timeintervals between the detected A1, A2 and A3 events or any combinationthereof. For example, the A2 sensing window 422 may be set to startbased on time intervals measured between a ventricular pacing pulse orsensed R-wave and the detected A1 event. The end of the A2 sensingwindow 422 may be set to start based on an A1-A2 time interval 432 orbased on an A1-A3 time interval 434. It is recognized that numerousmethods may be conceived for setting the A1, A2 and A3 sensing windows420, 422 and 424, respectively, based on the consistency of the expectedtime intervals between any combinations of the ventricular electricalevent (paced or sensed) and subsequent A1, A2 and A3 events.Furthermore, it is contemplated that these sensing windows 420, 422 and424 may be set according to different control parameters, such asdifferent fixed time intervals added to or subtracted from measuredevent time intervals depending on whether the ventricular electricalevent is a paced or sensed event and/or depending on heart rate. Theevent time intervals that may be measured and used for setting theonset, offset and duration of the sensing windows 420, 422 and 424 mayinclude any of the Vpace-A1, Vpace-A2, Vpace-A3, Rsense-A1, Rsense-A2,Rsense-A3, A1-A2, A1-A3, and/or A2-A3 time intervals determined during apaced and/or intrinsic rhythm.

The sensing threshold amplitudes 440, 442 and 444 may be set uniquelyduring each of the respective sensing windows 420, 422 and 424,respectively, or set to a fixed value for all sensing windows. Thesensing threshold amplitudes 440, 442, and 444 may be fixed or decayingthresholds and may be automatically adjusted thresholds set to startingthreshold values based on the peak motion sensor signal amplitudedetected during each respective window 420, 422 and 424. The motionsensor signals 400 and 410 are shown as raw signals, but the motionsensor signal may be filtered, amplified and rectified by circuitryincluded in motion sensor 212 to provide control circuit 206 with arectified signal that is used to detect the A1 through A4 events.

A post-ventricular, atrial refractory period 436 may be set based on theA1-A3 time interval 434 or based on the Vpace-A3 time interval (sum ofVpace-A1 interval 430 and A1-A3 time interval 434). In some examples,the atrial refractory period 436 ends upon the expiration of the A3sensing window 424. In other examples, the atrial refractory period 436ends after the expiration of the A3 sensing window 424. The A4 event 408or 418 may be detected in response to the first positive-going crossingof an A4 sensing threshold amplitude 446 by the rectified motion sensorsignal.

In some examples, the A4 detection is confirmed when the A1, A2 and A3events have each been detected during the atrial refractory period 436.If any one of the A1, A2 or A3 events was not detected during the atrialrefractory period 436, the A4 event detection based on a crossing ofthreshold 446 may not be confirmed and not used for starting an AVpacing interval. In other examples, at least one of the A1, A2 or A3events may be required to be detected during a respective sensing window420, 422, or 424 on a beat-by-beat basis for confirming an A4 detectionafter the atrial refractory period 436.

The A1, A2 and/or A3 events sensed during the respective A1 sensingwindow 420, A2 sensing window 422 and A3 sensing window 424 may be usedfor updating the atrial refractory period 436 as described inconjunction with FIG. 5 on a beat-by-beat or less frequent basis withoutrequiring positive detection of each of A1, A2, and/or A3 for confirmingan A4 detection on each beat. Setting the atrial refractory period basedon detection and relative timing of the A1 through A3 events enables theatrial refractory period to be set based on the consistent timing of theventricular motion sensor signal events so that A4 events may bedetected with high reliability even when the timing of the A4 eventrelative to the A1-A3 events and the preceding ventricular electricalevent is variable.

FIG. 7 is an averaged motion sensor signal 500 that may be determined bycontrol circuit 206 by averaging the motion sensor signal obtained overmultiple cardiac cycles, e.g., signals 400 and 410 of FIG. 6. Theaveraged motion sensor signal 500 may represent the average of 3 to 20or other predetermined number of cardiac cycles. The raw motion sensorsignal or a filtered, amplified and/or rectified motion sensor signalmay be buffered beginning from a ventricular electrical event, pacingpulse or sensed R-wave, at time 0.0 seconds until the next ventricularelectrical event. The buffered motion sensor signal obtained over onecardiac cycle may be averaged with the buffered motion sensor signalsobtained over a predetermined number of other cardiac cycles to produceaveraged motion sensor signal 500.

A ventricular electrical signal 510 is shown aligned in time withaveraged motion sensor signal 500. Ventricular electrical signal 510 maybe passed from sensing circuit 204 to control module 206 and includes anR-wave 512, which may be an evoked or intrinsic R-wave, and a T-wave514. R-wave 512 is followed by the ventricular contraction A1 event 502.The ventricular relaxation A2 event 504 occurs during T-wave 514. Thepassive ventricular filling A3 event 506 occurs after T-wave 514.

Since the A1, A2 and A3 events are ventricular mechanical events, theyoccur at consistent time intervals relative to each other and relativeto ventricular electrical events (R-wave 512 and T-wave 514). As aresult, the signal-to-noise ratio of the A1 signal 502, A2 signal 504and A3 signal 506 is improved in the averaged motion sensor signal 500compared to the single-cycle motion sensor signals 400 and 410 of FIG.6. The averaged A1 event 502, A2 event 504 and A3 event 506 have animproved signal-to-noise ratio compared to the A1, A2 and A3 eventsobserved in the motion sensor signal 400 or 410 of a single cardiaccycle as shown in FIG. 6, making A1, A2, and A3 event detection from theaveraged motion signal 500 more reliable. A single event detectionthreshold amplitude 540 may be defined such that the firstpositive-going crossing of the threshold 540 by the averaged, rectifiedmotion sensor signal 500 within the A1 sensing window 520, A2 sensingwindow 522 and A3 sensing window 524 is detected as the respective A1event 502, A2 event 504, and A3 event 506. Alternatively, uniquedetection threshold amplitudes may be defined for each sensing window520, 522 and 524 for detecting the respective A1, A2 and A3 events. Thesensing windows 520, 522 and 524 may be initially set according toexpected A1, A2 and A3 event timing following the ventricular pacingpulse or R-wave 512 and may be adjusted according to the actualdetection time of each respective A1 event 502, A2 event 504, and A3event 506. The sensing windows 520, 522 and 524 may be set based onventricular pacing rate or atrial event rate, e.g., based on A4-A4 eventintervals. The sensing windows 520, 522 and 524 may also be setdifferently following a ventricular pacing pulse than following anintrinsic R-wave sensed event since the timing of the A1, A2 and A3events and T-wave 514 may be altered during ventricular pacing comparedto during an intrinsic ventricular rhythm.

The atrial systolic A4 event timing, which is independent of theventricular electrical event timing, may be more variable from onecardiac cycle to the next with respect to the ventricular electrical andmechanical events, e.g., as shown by the relative timing of the A4events 408 and 418 of signals 400 and 410 (FIG. 6). As a result, the A4signal is largely attenuated in the averaged motion signal 500 in FIG.7. The improved signal-to-noise ratio of the A1 through A3 events andattenuation of the A4 event in the averaged motion signal 500 enablescontrol circuit 206 to reliably detect the signal averaged A1 event 502,A2 event 504 and A3 event 506 for determining one or more ventricularevent time intervals for use in setting A1, A2 and A3 detection windows420, 422, and 424, respectively, setting detection threshold amplitudesfor detecting the A1, A2, A3 and/or A4 events, and/or setting atrialrefractory period 436 used on a beat-by-beat basis for A4 eventdetection as shown in FIG. 6.

For example, a ventricular R-wave or pacing pulse to A1 time interval530, an A1-A3 time interval 534, A1-A2 time interval 536, a ventricularR-wave or pacing pulse to A3 time interval 516, and/or a T-wave to A3time interval 518 may be determined by control circuit 206 from theaveraged motion signal 500 and the cardiac electrical signal 510. Theatrial refractory period 436 is started upon delivering a ventricularpacing pulse or sensing an intrinsic R-wave. The atrial refractoryperiod 436 may be set to expire after a predetermined time interval,e.g., 30 to 100 ms, after the A3 time interval 516. For instance, iftime interval 516 is 700 ms, the atrial refractory period 436 may be setto expire 750 ms after the ventricular pacing pulse or sensed R-wavethat started the atrial refractory period. Instead of using a timeinterval ending with the A3 event detection, a time interval ending withthe A2 event detection may be determined and used in controlling theduration of the atrial refractory period 436. As described above, the A2event, which occurs during T-wave 514, is an indicator of the end ofventricular mechanical systole and the onset of ventricular mechanicaldiastole. The A3 event occurs during ventricular mechanical diastole,during the passive ventricular filling phase. As such the timing of theA2 event 504 or the timing of the A3 event 506 relative to anotherventricular electrical event (ventricular pacing pulse, R-wave 512, orT-wave 514) may be used for controlling the duration and expiration timeof atrial refractory period 436. In other words, the timing of aventricular mechanical diastolic event, A2 event 504 or A3 event 506,may be determined and used to set the atrial refractory period 436 thatis applied on a beat-by-beat basis for detecting A4 events.

The T-wave 514 may be sensed by sensing circuit 206 on a beat-by-beatbasis by control circuit 206 or by sensing circuit 204 from cardiacelectrical signal 510. The T-wave 514 may be sensed at a maximum peakamplitude of a rectified cardiac electrical signal or a maximum absolutepeak amplitude in a non-rectified cardiac signal received by controlcircuit 206 from sensing circuit 204. Alternatively, T-wave 514 may besensed by sensing circuit 204 in response to the cardiac electricalsignal crossing a T-wave sensing threshold amplitude after theventricular pacing pulse or R-wave sensed event signal. In some cases, aT-wave sensing window may be applied after the R-wave sensed eventsignal or a delivered pacing pulse to facilitate T-wave sensing.

The T-wave 514 may be sensed during the atrial refractory period 436.Control circuit 206 may terminate the atrial refractory period 436 at apre-determined time interval after sensing T-wave 514. For instance ifthe T-wave to A3 time interval 518 is determined to be 150 ms from theaveraged motion signal 500, control circuit 206 may terminate the atrialrefractory period 436 at 180 ms after sensing the T-wave to promotereliable sensing of the A4 event.

Atrial event detector circuit 240 may be a processor-based circuit thatdetermines the averaged motion sensor signal 500 over multiple cardiaccycles, detects A1, A2 and A3 events 502, 504, and 506 from the averagedmotion sensor signal 500, and sets the atrial refractory period 436based on the timing of at least one ventricular mechanical diastolicevent, e.g., the A3 event 506, detected from the average motion sensorsignal 500. In other examples, the A2 event is used as a ventriculardiastolic mechanical event for marking the approximate timing of theonset of ventricular diastole. The A4 event, e.g., event 408 or 418(FIG. 6) may be detected on a beat-by-beat basis from the non-averagedmotion sensor signal after the atrial refractory period 436 expires.

FIG. 8 is a flow chart 600 of a method performed by pacemaker 14 fordetecting atrial events and controlling atrial-synchronized ventricularpacing according to another example. At block 602, a ventricularelectrical event is identified, which may be a sensed intrinsic R-waveor delivered ventricular pacing pulse. A lower rate pacing interval maybe set at block 602 upon identifying the ventricular electrical event,as described in conjunction with FIG. 5, in order to maintain a minimum,base ventricular rate in the absence of A4 event detections.

At block 604, the motion sensor signal is buffered over the cardiaccycle, e.g., until the next ventricular electrical event is identified.At block 606, the buffered motion signal is averaged with bufferedmotion sensor signals acquired over a predetermined number of cardiaccycles to obtain an averaged motion signal with improved A1, A2 and A3signal-to-noise ratio and attenuated A4 signal compared to thenon-averaged motion sensor signal.

At block 608 the A1-A3 time interval or a ventricular electrical eventto A3 time interval is determined from the averaged motion sensor signalby detecting the signal averaged A1, A2 and A3 events as described abovein conjunction with FIG. 7. The A3 time interval is used to set theatrial refractory period at block 610 by atrial event detector circuit240. As described above, the atrial refractory period may be set apredetermined percentage or fixed time interval longer than the A1-A3time interval or a ventricular electrical event to A3 time interval orset to expire upon expiration of an A3 sensing window that is definedbased on relative timing of the A1, A2, and A3 events. In otherexamples, an A2 time interval is determined at block 608 for use insetting the A4 refractory period. The A2 and A3 events are ventricularmechanical diastolic event markers that may be used for controlling thetiming of the expiration of the A4 refractory period to occur near thestart or during the ventricular passive filling phase, before the activeventricular filling phase associated with atrial systole.

The atrial refractory period is started at block 610 upon identifying aventricular electrical event (pacing pulse or R-wave sensed event) atblock 602. In some examples, signal averaging and determination of theA3 time interval (or A2 time interval) for setting the atrial refractoryperiod may occur on a beat-by-beat basis using an averaged motionsignal. In other examples, the A3 time interval is determinedperiodically or in response to a change in the atrial rate, e.g.,determined from A4-A4 intervals, or a change between a sensed and pacedventricular rhythm. The most recently updated A3 time interval (or A2time interval) determined from the averaged motion sensor signal may beused to set the atrial refractory period at block 610. The expiration ofthe atrial refractory period may be set on the fly during an alreadystarted atrial refractory period based on the A3 time intervaldetermined during the current ventricular cycle. In other examples, theA3 time interval determined on a preceding ventricular cycle is used toset the atrial refractory period for the current ventricular cycle sothat the atrial refractory period ends during or after an expected timeof the A3 event, or in some cases prior to an expected A3 event butafter an expected A2 event.

In other examples, the duration of the atrial refractory period may becontrolled on a beat-by-beat basis by starting the atrial refractoryperiod upon the identified ventricular event, sensing the T-wave duringthe atrial refractory period, and terminating the atrial refractoryperiod a predetermined time interval after the sensed T-wave, where thepredetermined time interval is based on the T-wave to A3 time interval518 determined from the averaged motion signal 500 (FIG. 7).

If an A4 event is detected from the non-averaged motion sensor signal atblock 612, after the atrial refractory period expires, an AV pacinginterval is set at block 614. The A4 event may be detected based on anA4 detection threshold amplitude crossing by the raw motion sensorsignal or by the rectified signal. The pace timing circuit 242 sets anAV pacing interval at block 614 in response to the detected A4 signal.If an intrinsic R-wave is not sensed before expiration of the AV pacinginterval, as determined at block 616, the scheduled ventricular pacingpulse is delivered at block 620. In some cases, the A4 event may not bedetected before a lower rate pacing interval expires at block 615. Anatrial-asynchronous ventricular pacing pulse may be delivered at block620 if the lower rate pacing interval expires before an A4 event isdetected to maintain a programmed minimum ventricular base rate, causingthe process to return to block 602 where the ventricular pacing pulse isidentified as the next ventricular electrical event.

FIG. 9 is a flow chart 800 of a method for detecting A4 events forcontrolling atrial-synchronized ventricular pacing. At block 802, aventricular electrical event (ventricular pacing pulse or sensedintrinsic R-wave) is identified. An atrial refractory period is set atblock 804. During the A4 refractory period, detection of an atrialsystolic event, the A4 event, from the motion sensor is withheld ordisabled. In this example, the A4 refractory period is set based on thetiming of the A1, A2 and/or A3 events such that the A4 refractory periodexpires before the expected A3 event instead of after it as described inconjunction with FIG. 8. As heart rate increases, the time intervalbetween the A3 and A4 events may shorten, and in some instances the A3and A4 event signals become fused and appear as a single peak or becomeindistinguishable in the motion sensor signal. When fusion of the A3 andA4 event signals occurs, a single relatively larger amplitude signal mayoccur rather than the two relatively lower amplitude A3 and A4 eventsignals temporally separated at distinctly different times following theventricular electrical event as shown in FIG. 4.

As such, the A4 refractory period in the process of flow chart 800 isset at block 804 to allow sensing of the A4 event at some point afterthe expected A2 event but not necessarily later than the expected A3event. A higher A4 detection threshold amplitude, however, may be usedduring an expected time interval of the A3 event. The A4 detectionthreshold amplitude may be set to a starting threshold at block 806 thatis greater than the A4 detection threshold amplitude that is used afteran expected time of the A3 event such that only a high amplitude motionsensor signal representing the fused A3 and A4 events can be detectedduring an expected time interval of the A3 event. The A4 detectionthreshold amplitude starts at an initially high level at block 806 uponexpiration of the relatively shorter A4 refractory period, and athreshold adjustment interval is set at block 808.

The threshold adjustment interval may be a decay time or a drop timeinterval used to time the adjustment of the A4 detection thresholdamplitude to a second lower level after the expected time of the A3event. The A4 detection threshold amplitude may decay from the startinghigh level over a predetermined decay interval or make a stepwise dropfrom the starting high level to a second lower level after apredetermined drop time interval has expired. The threshold adjustmentinterval may be set based on the expected timing of the A3 event. An A3time interval may be determined as described previously herein, and theadjustment interval may be set at block 808 to expire a predeterminedtime interval later than the A3 time interval. In other examples, thethreshold adjustment interval may correspond to an A3 window, e.g.,window 524 of FIG. 7, determined from the averaged motion sensor signaland may be a time interval during which the A3 event is expected tooccur.

If the motion sensor signal crosses the A4 detection threshold at block810, the control circuit 206 sets an AV pacing interval at block 818. Ifthe threshold adjustment interval expires before the A4 event isdetected, “yes” branch of block 812, the A4 detection threshold isadjusted at block 814. The A4 detection threshold may be adjusted bychanging from a decaying threshold to a fixed threshold amplitude thatis lower than the starting threshold amplitude set at block 806. The A4detection threshold may alternatively be adjusted by dropping from thestarting threshold to a second, lower threshold amplitude in a stepchange. The A4 detection threshold may remain at the fixed lowerthreshold amplitude until an A4 event is detected (or a lower pacingrate interval expires) or may decay at the same or a different decayrate to a predetermined minimum A4 detection threshold amplitude. Inother examples, the A4 detection threshold may decay at a fixed ratefrom the starting threshold set at block 806 until an A4 event isdetected without setting or using a threshold adjustment interval. Ineach of these examples, the A4 detection threshold remains at agenerally higher amplitude during the expected time of the A3 event andfalls to a lower amplitude after the expected time of the A3 event.

The control circuit 206 sets the AV pacing interval at block 818 inresponse to detecting the A4 event at block 816. If an R-wave is sensedat block 820 during the AV pacing interval, it is identified as the nextventricular electrical event at block 802 and the process is repeated.If an intrinsic R-wave is not sensed during the AV pacing interval, thescheduled ventricular pacing pulse is delivered by pacemaker 14 at block822 upon expiration of the AV pacing interval. The pacing pulse isidentified as the next ventricular electrical event at block 802, andthe process is repeated for detecting the next A4 event during the nextventricular cycle.

While not shown explicitly in FIG. 9, it is contemplated that a back-uppacing interval or lower rate pacing interval may be set uponidentifying the ventricular electrical event at block 802. If the A4event is not detected before expiration of the back-up or lower ratepacing interval, a ventricular pacing pulse may be delivered that is nottracked to a detected A4 event. The use of a lower rate pacing intervalset upon identifying the ventricular electrical event for maintaining aminimum ventricular rate in the absence of a detected A4 event isdescribed above in conjunction with FIG. 5 (blocks 302 and 315) and FIG.8 (blocks 602 and 615), and the use of the lower rate pacing intervalmay be combined with the process of FIG. 9. Furthermore, it is to beunderstood that if an intrinsic R-wave is sensed before an A4 event isdetected, the process of FIG. 9 may return to block 802 to detect the A4event on the next ventricular cycle.

FIG. 10 is a timing diagram 850 of a motion sensor signal 854 that maybe received by pacemaker 14. Distinct A3 events 856 and A4 events 858are observed following ventricular pacing pulses 851 during the firstthree ventricular cycles. If the paced or intrinsic atrial rateincreases, fusion of the A3 and A4 events may occur producing highamplitude motion sensor signals representing the fused A3/A4 events 880as observed on the next three ventricular cycles.

The control circuit 206 sets an A4 refractory period 860 that expiresbefore an A3 interval 862, which may be determined as the time intervalfrom a ventricular electrical event to the A3 event identified from anaveraged motion sensor signal as described above in conjunction withFIG. 7 and FIG. 8. The A4 refractory period 860 may extend from adelivered ventricular pacing pulse 851 (or sensed intrinsic R-wave)through the A1 and A2 events and expire before an expected time of theA3 event but after an expected time of the A2 event. In some examples,the atrial refractory period 860 is set to extend longer than an A2 timeinterval determined from the averaged motion sensor signal or throughoutand expiring with a previously determined A2 window.

Upon expiration of the A4 refractory period 860, the A4 detectionthreshold 870 is set to a starting amplitude 872. In this example, thestarting threshold amplitude 872 is held constant for a thresholdadjustment interval 876 then drops step-wise to a second, lowerthreshold amplitude 874. The threshold adjustment interval 876 may beequal to an A3 window representing the expected time window of the A3event. The motion sensor signal 854 crosses the second, lower thresholdamplitude 874 during the first three ventricular cycles, resulting in A4event detections of the non-fused A4 events 858 of the motion sensorsignal. An AV pacing interval 878 may be set in response to detectingeach of the A4 events 858 for timing delivering of the next ventricularpacing pulse 851. The AV pacing interval may be set to 100 ms or less,for example to 50 ms, to provide desired synchrony between the atrialsystolic A4 event and the subsequent electrical depolarization of theventricle.

The fused A3/A4 events 880 are detected when the motion sensor signal854 crosses the first higher A4 detection threshold amplitude 872. Thecontrol circuit 206 may set an AV pacing interval based on the fusedA3/A4 event detections. In some examples, the AV pacing interval 878 maybe modified when the motion sensor signal crosses the higher thresholdamplitude 872 during the threshold adjustment interval 876 compared towhen the A4 event is detected based on a crossing of the second lowerthreshold amplitude 874. The AV pacing interval 878 may be adjusted inorder to promote separation of the A3 and A4 events. For example, the AVpacing interval 878 may be shortened so that the A3 event occurs earlierin the subsequent ventricular pacing cycle to separate the A3 event fromthe A4 event.

In FIG. 10, the A4 refractory period 860 extends through an expected A2event time but expires before an expected A3 event time. The A4detection threshold 870 set upon expiration of the A4 refractory periodstarts at a high level 872 and drops to a second, lower level 874 afterthe threshold adjustment interval 876 that extends after the expected A3event time. In other examples, the A4 refractory period 860 may beshorter, for example extending through the expected time of the A1event, but expiring before the expected time of the A2 event. In stillother examples, the A4 refractory period 860 is set to zero (or not setat all). In each of these instances, the A4 detection threshold 870 isset to an initially high level that is decreased, e.g., at apredetermined decay rate, slope or in one or more stepwise drops, to asecond lower detection threshold amplitude at some point after theexpected time of the A3 event such that only a high amplitude signalrepresentative of a fused A3/A4 event signal can be detected during thetime from the ventricular electrical event through an expected time ofthe A3 event.

The first higher level threshold amplitude 872 and the second lowerlevel threshold amplitude 874 may be predetermined values or set basedon peak amplitudes determined from the motion sensor signal. Forexample, the starting higher level threshold amplitude 872 may be setbased on a peak amplitude of the A1 event, A3 event, A4 event, or fusedA3/A4 event. For instance, when a fused A3/A4 event is detected, duringthe threshold adjustment interval 876, the peak amplitude of the fusedA3/A4 event may be determined. The starting, higher level thresholdamplitude 872 may be set to a percentage of the peak amplitude of thefused A3/A4 event on the next ventricular cycle.

In another example, the peak A1, A2 and/or A3 amplitudes are determinedfrom the motion sensor signal 854 for an individual cardiac cycle or anaveraged motion sensor signal determined by aligning and averagingmultiple ventricular cycles, e.g., averaged signal 500 of FIG. 7. Thestarting, higher threshold amplitude 872 may be set based on the A1, A2and/or A3 amplitudes so that the A4 detection threshold 870 remainsabove an expected A3 peak amplitude through the expected time of the A3event.

FIG. 11 is a flow chart 801 of a method for controlling atrialsynchronized ventricular pacing by pacemaker 14 according to anotherexample. Blocks 802, 804, 806, 808 and 810 in FIG. 11 correspond toidentically-numbered blocks described above in conjunction with FIG. 9.In the process shown in FIG. 11, if the A4 event is detected during thethreshold adjustment interval at block 810, in response to a higherthreshold amplitude crossing, the detected A4 event is likely a fusedA3/A4 event as described in conjunction with FIG. 10. At block 815,control circuit 206 may set the AV pacing interval to an adjustedinterval in response to detecting the fused A3/A4 event during thethreshold adjustment interval. The adjusted AV pacing interval may beshortened from the target AV pacing interval set when the A4 event isdetected after the threshold adjustment interval and is separated fromthe A3 event in time. For example, if the target AV pacing interval is250 to 300 ms, the adjusted AV pacing interval may be shortened by up to100 ms to separate the A3 and A4 events. When the ventricular pacingpulse is delivered earlier after the A4 event, at a shorter AV pacinginterval, the A3 event occurs earlier in the subsequent ventricularcycle since it is a ventricular event (representing passive ventricularfilling) and therefore follows the earlier timing of the ventricularpacing pulse.

If the A4 event is detected at block 816 in response to a motion sensorsignal crossing of the adjusted lower threshold amplitude (block 814),after the threshold adjustment interval expires (bock 812), the AVpacing interval is set at block 818 to the target AV interval. Thetarget AV interval may be determined to optimize atrioventricularsynchrony at relatively lower heart rates or when clear temporalseparation of the atrial and ventricular motion sensor signals ispresent. Blocks 812, 814, 816, 818, 820 and 822 correspond toidentically-numbered blocks described above in conjunction with FIG. 9.

FIG. 12 is a flow chart 900 of another example of a method forcontrolling atrial-synchronized ventricular pacing by pacemaker 14. Aventricular electrical event is identified at block 902. In response toidentifying the ventricular electrical event, the control circuit 206sets the A4 refractory period at block 903 to a “long” A4 refractoryperiod that expires after the expected time of the A3 event so that theA4 event is only detected after an expected time of the A3 eventfollowing the identified ventricular event.

During the A4 refractory period, the motion sensor signal may bemonitored for identifying the A3 event signal and determining it peakamplitude at block 904. If the A3 and A4 events become fused, a largeamplitude signal may occur during the long A4 refractory period, e.g.,during an A3 window 424 set during the A4 refractory period as shown inFIG. 6. A maximum absolute peak amplitude of the motion sensor signalduring the long A4 refractory period and its timing during the A4refractory period, or a maximum amplitude specifically during the A3window, may be determined for detecting fusion of the A3 and A4 eventswhen the A4 event is not detected outside the A4 refractory period asfurther described below.

After expiration of the A4 refractory period, the A4 detection thresholdis set at block 905 to a relatively low threshold amplitude, e.g.,corresponding to the second, lower level threshold amplitude 874 shownin FIG. 10. The low threshold amplitude may be greater than the expectedamplitude of the A3 events but less than the expected amplitude of theA4 events. Control circuit 206 waits for the motion sensor signal tocross the A4 detection threshold at block 906. If a ventricularelectrical event occurs before an A4 event is detected, as determined atblock 908, control circuit 206 may determine a peak amplitude of themotion sensor signal at block 910. If a large peak amplitude signaloccurred during the expected A3 event time, within the A4 refractoryperiod or within an A3 window, fusion of the A3 and A4 events may bedetected. A large peak amplitude signal may be detected when the motionsensor signal crosses a predetermined fusion detection threshold priorto expiration of the A4 refractory period. The fusion detectionthreshold may be set to a higher threshold amplitude, e.g., thresholdamplitude 872 shown in FIG. 10, which is greater than the A4 detectionthreshold amplitude set at block 906 after expiration of the long A4refractory period. Control circuit 206 may be configured to detectfusion by determining a peak amplitude of the motion sensor andcomparing the peak amplitude to a fusion detection threshold when themotion sensor signal amplitude does not cross the A4 detection thresholdamplitude after the A4 refractory period.

In some examples, the motion sensor signal is buffered in memory 210during the long A4 refractory period, or only during an A3 window, toenable detection of a large amplitude signal at block 910 only if an A4event is not detected at block 906 before the next ventricularelectrical event. The ventricular electrical event may be a sensedintrinsic R-wave or a back-up ventricular pacing pulse delivered if aback-up or lower rate pacing interval has expired without detecting anA4 event or sensing an intrinsic R-wave.

If a large amplitude signal during the A4 refractory period is notdetected at block 910, indicating unlikely fusion of the A3 and A4events, control circuit 206 may adjust A4 detection parameters at block912. For example, the A4 detection threshold may be reduced and/or theA4 refractory period may be shortened. In some examples, control circuit206 may repeat an analysis of the averaged motion sensor signal 500 asshown in FIG. 7 to re-determine expected timing and amplitude of the A1,A2 and/or A3 events. The relative timing of the ventricular mechanicalevents to the each other and/or the preceding ventricular electricalevent and their respective amplitudes may be used for adjusting the A4refractory period and/or A4 detection threshold amplitude to promote A4event detection.

If a large amplitude signal is detected at block 910, fusion of the A3and A4 events is indicated. At block 916, control circuit 206 enablesfused A3/A4 event detection. For example, fused A3/A4 event detectionmay be enabled by adjusting the A4 refractory period to a relativelyshorter interval, such as the A4 refractory period 860 of FIG. 10 thatexpires before the expected A3 event time. Fused A3/A4 event detectionmay be enabled by allowing A4 events to be detected during the long A4refractory period if the motion sensor signal crosses a high A4detection threshold amplitude during the A4 refractory period. In someexamples, fused A3/A4 event detection is enabled by setting the A4detection parameters according to the techniques shown and described inconjunction with FIG. 10 which use a short A4 refractory period 860 anda variable A4 detection threshold 870 controlled using a thresholdadjustment interval 876. Any of the other techniques described above inconjunction with FIG. 10 for detecting a fused A3/A4 event signal duringan expected A3 event time may be enabled at block 916.

Pacemaker 14 operates using the enabled fused A3/A4 detection controlparameters for detecting A4 events and setting the AV pacing intervalfor delivering atrial synchronized ventricular pacing pulses. While thefused A3/A4 detection control parameters are enabled, however, controlcircuit 206 may monitor detected A4 events to detect an indication ofnon-fused A4 events. The A3 and A4 event signals may separate due to achange in heart rate. Control circuit 206 may monitor for separation ofthe A4 event signal from the A3 event signal at block 918 while thefused A3/A4 event detection control parameters are enabled, so thatpacemaker 14 can switch back to the long A4 refractory period and lowerA4 detection threshold amplitude for detecting A4 events when the A3 andA4 events are no longer fused.

An indication of a non-fused A4 event may be detected at block 918 inresponse to an A4 event being detected later than the expected A3 eventtime, after a threshold adjustment interval, and/or in response to themotion sensor signal crossing a relatively low A4 detection thresholdamplitude for one or more ventricular cycles. For instance, using theexample techniques of FIG. 10, if an A4 detection is made after thethreshold adjustment interval 876, which is later than an expected A3event time and based on the lower detection threshold amplitude 874,control circuit 206 detects an indication of a non-fused A4 event atblock 918. In some examples, control circuit 206 may detect anindication of non-fused A4 events when A4 events are detected after thethreshold adjustment interval 876 consistently for a predeterminednumber of ventricular cycles, e.g., at least 3 consecutive ventricularcycles.

If an indication of non-fused A4 events is detected at block 918,control circuit 206 may disable fused A3/A4 detection at block 920 byswitching back to setting the “long” A4 refractory period that expiresafter an expected A3 event time and setting the A4 detection thresholdamplitude back to a relatively low threshold amplitude beginning fromthe expiration of the A4 refractory period. The process returns to block902 to identify the next ventricular electrical event and detect A4events according to the detection control parameters set at blocks 903and 905.

When the A4 events are detected outside the A4 refractory period, “yes”branch of block 906, control circuit 206 may be configured to monitorthe motion sensor signal for detecting an indication of A3-A4 intervalshortening at block 914. If A3 events are being detected during an A3window (during the long A4 refractory period), the A3-A4 time intervalmay be determined directly at block 914. One or more A3-A4 timeintervals may be required to be less than a threshold time interval orsuccessively decreasing by a threshold amount, e.g., compared to apreceding A3-A4 time interval, for detecting the indication of A3-A4interval shortening at block 914.

In other examples, indirect metrics that indicate that the A3-A4interval may be shortening may be determined at block 914. For example,A4-A4 intervals may be determined and if the A4-A4 intervals aredecreasing, indicating an increase in atrial rate, an indication ofA3-A4 interval shortening may be detected at block 914. In anotherexample, at time interval from the ventricular electrical event or theA1 event to the A2 or A3 event may be determined at block 914. If thetime interval between the ventricular electrical event or the A1ventricular systolic mechanical event to the subsequent A2 or A3ventricular diastolic mechanical events is shortening, this may be anindication of A3-A4 interval shortening.

If an indication of A3-A4 interval shortening is not detected, “no”branch of block 914, control circuit 206 continues to detect A4 eventsbased on the detection control parameters set at blocks 903 and 905. Ifan indication of shortening is detected at block 914, the fused A3/A4detection control parameters may be enabled at block 916 in anticipationthat the A3 and A4 events may become fused on subsequent ventricularcycles. In some examples, when an indication of A3-A4 intervalshortening is detected at block 914, the AV pacing interval may beadjusted at block 915, e.g., shortened from the AV pacing interval setwhen A3-A4 interval shortening is not being detected, to increaseseparation of the A3 and A4 events in addition to or alternatively toenabling the fused A3/A4 detection control parameters at block 916. Ifthe A3-A4 time interval is determined to increase or lengthen again, oran indirect indicator of a lengthening of the A3-A4 time interval isdetermined, an indication of non-fused A4 events may be detected atblock 918. The control circuit may disable the fused A3/A4 detectioncontrol parameters at block 920.

While not shown explicitly in FIG. 12, it is to be understood thatthroughout the operation of control circuit 206 for detecting A4 eventsusing the detection control parameters set at blocks 903 and 905 orusing fused A3/A4 detection control parameters enabled at block 916,pace timing circuit 242 sets the AV pacing interval in response todetected A4 events for controlling pulse generator 202 to deliverventricular pacing pulses in an atrial synchronized pacing mode.Adjustments to the AV pacing interval may occur in response to detectingfused A3/A4 events or an indication of A3-A4 time interval shortening orboth to promote an increased separation of the A3 and A4 events.Furthermore, it is understood that if A4-A4 event intervals becomeshorter than an atrial tracking limit, indicating the atrial rate isfaster than a desired maximum tracking rate, pacemaker 14 may switch toa non-atrial tracking pacing mode. AV pacing intervals are not set inresponse to A4 event detection in this situation. Pace timing circuit242 may set lower rate pacing intervals (VV pacing intervals) tomaintain a minimum ventricular rate by delivering ventricular pacingpulses upon expiration of the lower rate pacing intervals, asynchronousto the atrial events.

FIG. 13 is a flow chart 950 of a method for controlling ventricularpacing by pacemaker 14 according to another example. At block 952, aventricular electrical event is identified, and the subsequent A3 and A4events are detected at block 954 using any of the techniques describedabove. The A3-A4 time interval is determined at block 956. If A3-A4interval shortening is not detected (block 958) based on a comparison toa previous A3-A4 time interval or to a shortening threshold interval,pace timing circuit 242 sets the AV pacing interval at block 960 tocontrol pulse generator 202 to deliver the next ventricular pacing pulseat a target AV pacing interval from the A4 event.

A decreasing trend of the A3-A4 time interval may be detected at block958, for example in response to shortening of the A3-A4 time intervalcompared to a preceding A3-A4 time interval or a predetermined number ofconsecutively decreasing A3-A4 time intervals. If A3-A4 shortening isdetected, the AV pacing interval is adjusted at block 962. The AV pacinginterval may be shortened by control circuit 206 to promote temporalseparation of the A3 and A4 events by controlling pulse generator 202 todeliver the next ventricular pacing pulse at block 964 earlier after theA4 event than the target AV pacing interval used at block 960. Theearlier ventricular pacing pulse causes the A3 event to occur earlier inthe next ventricular cycle, ahead of the next A4 event.

At block 966, the A3-A4 time interval after the earlier ventricularpacing pulse delivered at the shortened AV pacing interval isdetermined. The A3-A4 time interval may be compared to a threshold timeinterval at block 968 to determine if acceptable separation of the A3and A4 events has occurred. If the A3-A4 time interval is acceptable atblock 968, the AV pacing interval may be maintained at the adjustedinterval unless further A3-A4 time interval shortening is detected atblock 958. It is to be understood that the AV pacing interval may beshortened in response to detecting a shortening of the A3-A4 interval upto a predetermined maximum number of times or down to a minimum allowedAV pacing interval.

In some examples, control circuit 206 may periodically increase the AVpacing interval at block 970 when the A3-A4 interval is acceptable atblock 968 to determine if the AV pacing interval can be increased againwhile still maintaining separation of the A3-A4 interval. If the A3-A4interval is not less than an acceptable time interval threshold at block958, after increasing the AV pacing interval for one or more pacingcycles, the AV pacing interval may be adjusted back to the target AVpacing interval at block 960. Control circuit 206 may be configured tomonitor the A3-A4 time interval to maintain a maximum temporalseparation of the A3 and A4 events by adjusting the AV pacing intervalto the longest AV pacing interval that maintains a maximum or optimallyincreased A3-A4 time interval.

Various examples of an intracardiac pacemaker configured to deliveratrial-synchronized ventricular pacing have been described according toillustrative embodiments. The ventricular intracardiac pacemaker isconfigured to detect A4 events from a motion sensor signal forcontrolling the atrial-synchronized ventricular pacing according tovarious methods described above. The methods described herein andrepresented by the accompanying flow charts and timing diagrams maycombined or modified to include steps performed in a different order orcombination than the illustrative examples shown. Furthermore, othercircuitry may be conceived by one of ordinary skill in the art forimplementing the techniques disclosed herein; the particular examplesdescribed herein are illustrative in nature and not intended to belimiting. It is appreciated that various modifications to the referencedexamples may be made without departing from the scope of the disclosureand the following claims.

1. An intracardiac ventricular pacemaker, comprising: a pulse generatorconfigured to generate and deliver pacing pulses to a ventricle of apatient's heart via electrodes coupled to the pacemaker; a motion sensorconfigured to produce a motion signal comprising an atrial systolicevent and a ventricular diastolic event indicating a passive ventricularfilling phase; and a control circuit coupled to the motion sensor andthe pulse generator and configured to: set a detection threshold to afirst amplitude during an expected time interval of the ventriculardiastolic event and to a second amplitude lower than the first amplitudeafter the expected time interval of the ventricular diastolic event;detect the atrial systolic event in response to the motion signalcrossing the detection threshold; set an atrioventricular pacinginterval in response to detecting the atrial systolic event; and controlthe pulse generator to deliver a pacing pulse to the ventricle inresponse to the atrioventricular pacing interval expiring.
 2. Thepacemaker of claim 1, wherein the control circuit is configured toadjust the detection threshold from the first amplitude to the secondamplitude at a decay rate over the expected time interval of theventricular diastolic event.
 3. The pacemaker of claim 1, wherein thecontrol circuit is further configured to start an atrial refractoryperiod upon a ventricular electrical event and end the atrial refractoryperiod prior to the expected time interval of ventricular diastolicevent, wherein detection of the atrial systolic event is disabled duringthe atrial refractory period.
 4. The pacemaker of claim 1, wherein thecontrol circuit is configured to: set the atrioventricular pacinginterval to a first interval in response to detecting the atrialsystolic event after the expected time interval of the ventriculardiastolic event; set the atrioventricular pacing interval to a secondinterval in response to detecting the atrial systolic event during theexpected time interval of the ventricular diastolic event, the secondinterval shorter than the first interval; and control the pulsegenerator to deliver the ventricular pacing pulse upon expiration of theatrioventricular pacing interval.
 5. The pacemaker of claim 1, whereinthe control circuit is further configured to: detect a fusion of theventricular diastolic event and the atrial systolic event in response tothe motion signal crossing the detection threshold during the expectedtime interval of the ventricular diastolic event; and adjust theatrioventricular pacing interval in response to detecting the fusion. 6.The pacemaker of claim 1, wherein the control circuit is furtherconfigured to: set a first atrial refractory period to expire after theexpected time interval of the ventricular diastolic event during a firstventricular cycle, wherein detection of the atrial systolic event fromthe motion signal is withheld during the first atrial refractory period;set the detection threshold to the second amplitude after expiration ofthe first atrial refractory period; responsive to the motion signal notcrossing the second amplitude of the detection threshold, shorten thefirst atrial refractory period to a second atrial refractory period setto expire before the ventricular diastolic event during a secondventricular cycle; and set the detection threshold to the firstamplitude upon expiration of the shortened refractory period.
 7. Thepacemaker of claim 1, wherein the control circuit is further configuredto: set an atrial refractory period expiring after the expected timeinterval of the ventricular diastolic event during a first ventricularcycle, wherein detection of the atrial systolic event is inhibitedduring the atrial refractory period; detect an indication of shorteningof a time interval from the ventricular diastolic event of the motionsignal to the atrial systolic event of the motion signal; and enabledetection of the atrial systolic event during the time interval of theexpected ventricular diastolic event during a second ventricular cyclein response to detecting the indication of shortening.
 8. The pacemakerof claim 7, wherein the control circuit is configured to enabledetection of the atrial systolic event by shortening the atrialrefractory period to expire before the expected time interval of theventricular diastolic event and enabling the detection threshold to beset to the first amplitude after expiration of the shortened atrialrefractory period.
 9. The pacemaker of claim 1, wherein the controlcircuit is configured to: detect an indication of shortening of a timeinterval from the ventricular diastolic event of the motion signal tothe atrial systolic event of the motion signal; and adjust theatrioventricular pacing interval in response to detecting the indicationof shortening.
 10. The pacemaker of claim 1, wherein the control circuitis further configured to: set an atrial refractory period expiring afterthe expected time interval of the ventricular diastolic event; determineif the motion sensor signal exceeds a fusion detection threshold inresponse to the motion signal not crossing the detection threshold afterthe atrial refractory period; detect fusion of the atrial systolic eventand ventricular diastolic event in response to the motion sensor signalexceeding the fusion detection threshold; and enable detection of theatrial systolic event during the time interval of the expectedventricular diastolic event in response to detecting the fusion.
 11. Thepacemaker of claim 1, further comprising a housing enclosing the pulsegenerator, the motion sensor, and the control circuit, wherein theelectrodes are housing-based electrodes.
 12. A method performed by anintracardiac pacemaker having a motion sensor, comprising: producing bythe motion sensor a motion signal comprising an atrial systolic eventand a ventricular diastolic event indicating a passive ventricularfilling phase; setting by a control circuit of the pacemaker a detectionthreshold to a first amplitude during an expected time interval of theventricular diastolic event and to a second amplitude lower than thefirst amplitude after an expected time interval of the ventriculardiastolic event; detecting the atrial systolic event in response to themotion signal crossing the detection threshold; setting anatrioventricular pacing interval in response to detecting the atrialsystolic event; and delivering a pacing pulse to a ventricle of apatient's heart via electrodes coupled to the pacemaker in response tothe atrioventricular pacing interval expiring.
 13. The method of claim12, further comprising adjusting the detection threshold from the firstamplitude to the second amplitude at a decay rate over the expected timeinterval of the ventricular diastolic event.
 14. The method of claim 12,further comprising starting an atrial refractory period upon aventricular electrical event and ending the atrial refractory periodprior to the expected time interval of ventricular diastolic event,wherein detection of the atrial systolic event is disabled during theatrial refractory period.
 15. The method of claim 12, furthercomprising: setting the atrioventricular pacing interval to a firstinterval in response to detecting the atrial systolic event after theexpected time interval of the ventricular diastolic event; setting theatrioventricular pacing interval to a second interval in response todetecting the atrial systolic event during the expected time interval ofthe ventricular diastolic event, the second interval shorter than thefirst interval; and controlling the pulse generator to deliver theventricular pacing pulse upon expiration of the atrioventricular pacinginterval.
 16. The method of claim 12, further comprising: detecting afusion of the ventricular diastolic event and the atrial systolic eventin response to the motion signal crossing the detection threshold duringthe expected time interval of the ventricular diastolic event; andadjusting the atrioventricular pacing interval in response to detectingthe fusion.
 17. The method of claim 12, further comprising: setting afirst atrial refractory period to expire after the expected timeinterval of the ventricular diastolic event during a first ventricularcycle, wherein detection of the atrial systolic event from the motionsignal is withheld during the first atrial refractory period; settingthe detection threshold to the second amplitude after expiration of thefirst atrial refractory period; responsive to the motion signal notcrossing the second amplitude of the detection threshold, shortening thefirst atrial refractory period to a second atrial refractory period setto expire before the ventricular diastolic event during a secondventricular cycle and setting the detection threshold to the firstamplitude upon expiration of the shortened refractory period.
 18. Themethod of claim 12, further comprising: setting an atrial refractoryperiod expiring after the expected time interval of the ventriculardiastolic event during a first ventricular cycle, wherein detection ofthe atrial systolic event is inhibited during the atrial refractoryperiod; detecting an indication of shortening of a time interval fromthe ventricular diastolic event of the motion signal to the atrialsystolic event of the motion signal; and enabling the detection of theatrial systolic event during the time interval of the expectedventricular diastolic event during a second ventricular cycle inresponse to detecting the indication of shortening.
 19. The method ofclaim 18, wherein enabling detection of the atrial systolic eventcomprises shortening the atrial refractory period to expire before theexpected time interval of the ventricular diastolic event and enablingthe detection threshold to be set to the first amplitude afterexpiration of the shortened atrial refractory period.
 20. The method ofclaim 12, further comprising: detecting an indication of shortening of atime interval from the ventricular diastolic event of the motion signalto the atrial systolic event of the motion signal; and adjusting theatrioventricular pacing interval in response to detecting the indicationof shortening.
 21. The method of claim 12, further comprising: settingan atrial refractory period expiring after the expected time interval ofthe ventricular diastolic event; determining if the motion sensor signalexceeds a fusion detection threshold in response to the motion signalnot crossing the detection threshold after the atrial refractory period;detecting fusion of the atrial systolic event and ventricular diastolicevent in response to the motion sensor signal exceeding the fusiondetection threshold; and enabling detection of the atrial systolic eventduring the time interval of the expected ventricular diastolic event inresponse to detecting the fusion.
 22. The method of claim 12, whereinthe electrodes comprise housing based-electrodes, the method furthercomprising delivering the ventricular pacing pulse via the housing-basedelectrodes.
 23. A non-transitory computer-readable medium storing a setof instructions which when executed by a control circuit of anintracardiac ventricular pacemaker having a motion sensor, cause thepacemaker to: produce by the motion sensor a motion signal comprising anatrial systolic event and a ventricular diastolic event indicating apassive ventricular filling phase and; set a detection threshold to afirst amplitude during an expected time interval of the ventriculardiastolic event and to a second amplitude lower than the first amplitudeafter an expected time interval of the ventricular diastolic event;detect the atrial systolic event in response to the motion signalcrossing the detection threshold; set an atrioventricular pacinginterval in response to detecting the atrial systolic event; and delivera pacing pulse upon expiration of the atrioventricular pacing intervalto a ventricle of a patient's heart via electrodes coupled to thepacemaker.